Your best bet is not to memorize derived formulas. The momentum before the collision is mA*vA+mB*vB. The momentum after is (mA+mB)*vC where vC is the velocity of the wreckage. And they are equal. Remember that velocity has a sign depending on the direction of travel.

I didn't use a formula. I just remembered that momentum is equal to mass*velocity - and that the sum of the momenta before the collision is equal to the sum of the momenta after. Unfortunately, your answer is not correct. Remember that the collision is head-on. One of the velocities needs to be negative. They aren't both headed in the same direction!

No problem. There is one thing on the first page near the bottom that I noticed; you have the momentum terms as m+v. I'm guessing you just copied it wrong or something because you used it correctly later on.